It'll fit but the gearing is all wrong. With a high torque powerband like the TDI has, there's not much benefit to a 6 speed. What you're thinking of is like a double overdrive, most 6 speeds are single overdrive. A 6 speed is best for a performance engine with low torque. It can give better mileage but putting a gasoline engine from another VW won't do that.
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